Process for preparing propylene copolymers

ABSTRACT

There is provided a process for the copolymerisation of propylene with at least one C 4-20  α-olefin or C 4-20  diene in a reaction medium in the presence of a catalyst system at a temperature of greater that 80° C. or at a temperature and pressure above the critical temperature and pressure of the reaction medium. Under such conditions, comonmer incorporation is higher than under standard conditions.

This invention relates to a process for making propylene copolymers, inparticular to a process for making propylene copolymers with higherolefins at high temperatures, e.g. temperatures approaching the criticaltemperature of the reaction medium.

The polymerisation of propylene and comonomers in a loop reactor is awell known process. Typically, polymer is formed as solid particlessuspended in a liquid comprising mainly propylene monomer. The reactorcontent is maintained in a highly agitated state by circulating thereaction mixture at comparatively high velocity around the loop by meansof a pump. The heat of polymerisation is removed by a cooling jacketenclosing the reactor and polymer is removed from the reactorcontinuously or discontinuously.

There are however some problems associated with conventional loopreactor polymerizations. Firstly, the reactor temperature and pressuremust be such that the entire reactor is completely filled with thereaction mixture and no vapour bubbles are present. Secondly, thereaction medium needs to be carefully selected to ensure minimum polymersolubility, a particular problem with copolymers. Finally, the reactionmedium must also be volatile to allow its easy separation from thefinished polymer powder.

There are also problems associated with the manufacture of propylenecopolymers with higher olefins since comonomer incorporation of higherolefins is low.

It is well-known in the art that higher olefins, e.g. hexene or dieneolefins, polymerise much more slowly than lower alpha olefins such asethylene and propylene. Hence, in order to get a desired amount ofhigher olefin into a copolymer a considerable excess of higher olefin isrequired. Since only a very limited amount of the higher olefin actuallyreacts during the polymerisation, the residual unreacted monomers needto be removed, and separated from the other monomers and polymerproducts which is clearly an expensive, undesirable and technicallychallenging process, especially when large amounts of monomer need to beseparated. Moreover, any unreacted monomers remaining in the finalpolymer product cause undesirable odour and taste and may pose a healthhazard.

In order to alleviate some of the problems with loop reactorpolymerisation discussed above, EP-A-854887 suggests the use ofsupercritical conditions under which to carry out propylene homo orcopolymerisation especially with ethylene. By supercritical conditionsit is meant that both the temperature and the pressure within thereactor are above the corresponding critical temperature and thecritical pressure of the reaction medium (normally mainly propylene).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effective temperature on incorporation of1-hexene into the polymer. Examples 2, 5, 6 and Comparative Example 4provide the data points for this graph.

The applicant has surprisingly found that by copolymerising propylenewith higher olefins at high temperature, for example, undersupercritical conditions, the incorporation of higher olefin into apolymer is greater and hence there is less unreacted higher olefinremaining after polymerisation has been completed, hence less higherolefin needs to be removed in the process, separated and recirculated.As used herein, higher olefins include diene olefins and α-olefinshaving at least 4 carbon atoms. Moreover, due to the higherincorporation of higher olefins observed at high temperature or undersupercritical conditions, less starting higher olefin is required givingrise to considerable economic savings.

In addition, by using higher temperature, any unreacted higher olefinmay be more easily removed from the reactor since little additional heatis required to evaporate the higher olefin monomer.

Thus, viewed from one aspect the invention provides a process for thecopolymerisation of propylene with at least one C₄₋₂₀ α-olefin or C₄₋₂₀diene olefin in a reaction medium in the presence of a catalyst systemat a temperature of greater than 80° C. The copolymerisation preferablytakes place as a slurry or bulk polymerisation, preferably in a loopreactor.

Viewed from another aspect the invention provides a propylene copolymerwith a C₄₋₂₀ α-olefin or C₄₋₂₀ diene olefin obtained by a process ashereinbefore described.

The process of the invention requires operation at a temperature ofgreater than 80° C., preferably at least 85° C. Alternatively, theprocess of the invention may be operated under supercritical conditions.This latter condition means that the temperature in the reactor must behigher than the corresponding critical temperature of the reactionmixture and the pressure in the reactor higher than the correspondingcritical pressure of the reaction mixture.

The reaction medium is formed from the propylene and comonomers presentas well as any hydrogen or other adjuvant employed.

In a process for the manufacture of propylene copolymers as described inthe present invention, the major part of the reaction medium will beformed by propylene. The critical temperature and pressure of propylenewould be 91.4° C. and 46 bars. Hence, in order to carry out apolymerisation under supercritical conditions where propylene formed thetotality of the reaction medium, the temperature must exceedapproximately 92° C. and the pressure must exceed 46 bars.

However, in the present invention at least one higher olefin (i.e. aC₄₋₂₀ α-olefin or C₄₋₂₀ diene olefin) must be present. The presence ofsuch a higher olefin which by definition has a higher boiling point thanpropylene means that the critical temperature of the reaction mediumincreases. The size of the increase is, of course, dependent on thenature of the comonomer and the amount present in the reactor. Thetemperature and pressure conditions required to ensure that the reactiontakes place under supercritical conditions will be readily determined bythe skilled chemical engineer.

In the process of the invention, the reactor temperature must be greaterthan 80° C., preferably at least 85° C., more preferably at least 90°C., especially at least 100° C., e.g. 100 to 110° C. At such highpolymerisation temperatures (i.e. temperatures greater than 80° C.)incorporation of higher olefin into a copolymer is higher than underconventional lower temperature polymerisation conditions, e.g those atless than 80° C., typically 50 to 75° C.

The ultimate upper limit of the temperature within the reactor in anysituation is the melting point of the resulting copolymer butunnecessarily high temperatures also cause an increase in the solubilityof the polymer which is undesirable. The preferred temperature range istherefore between 90 to 105° C.

Where supercritical temperatures are employed, these are preferably inthe range of 92 to 120° C., particularly 92 to 110° C., especially 92 to100° C. Preferably, supercritical pressure ranges are from 46 to 100bars, preferably 50 to 70 bars when the reaction medium comprises mainlypropylene. But as discussed in detail above, the critical temperatureand pressure of a reaction medium depend not only on the propylenepresent but also on the amount of higher olefins used.

Under non-supercritical conditions, the pressure within the reactorvessel may be in the range 30 to 60 bars, e.g. 35 to 55 bars. Thepressure is controlled to ensure that the reaction mixture is maintainedin a liquid state.

In general, the temperature and pressure conditions required for theprocess of the invention can be summarised as follows:

-   1. Temperature>80° C., preferably>85° C., more preferably>90° C. but    less than T_(c) when the pressure in the reactor is greater or less    than the supercritical pressure of the reaction mixture.-   2. T>T_(c) when pressure sufficient to maintain the mixture in a    liquid state.-   3. Supercritical conditions, i.e. both temperature and pressure    being greater than T_(c) and p_(c) respectively.

Where large amounts of higher olefin comonomer are present, the criticaltemperature of the reaction medium may be so high that it exceeds themelting temperature of the polymer product or the solubility of thepolymer product becomes unacceptably high. In such a scenario, it maytherefore be necessary to adjust the critical temperature of thereaction medium by adding adjuvants to the reactor. Suitable adjuvantsare those of low molecular weight that when added to the reaction mediuminevitably cause the critical temperature of the medium to fall.Preferred adjuvants are methane and ethane. The addition of theseadjuvants can decrease the critical temperature of the reaction mediumdown to levels suitable for the polymerisation in question.

The critical temperature and pressure of methane is −82.1° C. and 45.8atm. The corresponding values for ethane are 32.3° C. and 48.2 atm.These components can be added to the reaction medium in amounts of up to20% by weight and can reduce the critical temperature of the reactionmedium to well below 90° C.

Physical properties of the reaction mixture such as the critical pointcan be calculated by using the equation of state. Some examples of theseequations are SRK (Soave-Redlich-Kwong) and Peng-Robinson (Theintroduction to Aspen plus, course notes, May 2000, Aspen Technology,Inc. page 86).

The skilled person will also realise that certain catalysts operate moreefficiently at less than 90° C. and hence reduction of temperature toimprove catalyst performance may also be required. Conversely, if thetemperature is too high, e.g. above 110° C., then the activity of thecatalyst may decrease.

In the process of the invention, propylene needs to be copolymerisedwith a C₄₋₂₀ diene or a C₄₋₂₀ α-olefin, e.g. C₅₋₂₀ α-olefin, C₆₋₁₆ orC₄₋₁₆ α-olefin, preferably C₆₋₁₀ α-olefin. Suitable dienes are C₄₋₁₀dienes, e.g. butadiene, 1,4-pentadiene or 1,5-hexadiene. Preferably, thecomonomer should be a C₄₋₁₀ α-olefin, e.g. C₅₋₁₀ α-olefin, such ashexene, 4-methylpentene, heptene or octene. In an especially preferredembodiment, the comonomer is a C₆₋₈ linear α-olefin, especially hexeneor octene.

The amount of higher olefin fed into the polymerisation process dependsvery much on the amount of the higher olefin desired in the polymerproduct. It is preferred however, if the higher olefin feed makes up nomore than 30 wt %, e.g. no more than 20 wt % of the feed into theprocess relative to propylene. Preferably, the amount of higher olefinin the reaction medium should be up to 15 wt %, e.g. in the range from 1to 15 wt %, especially up to 10 wt % relative to the amount of propyleneemployed. Even small changes in the amount of higher olefin fed into thepolymerisation process can significantly affect the amount of higherolefin incorporated. Moreover, small changes in the amount of comonomerincorporated can have significant impact on polymer properties.

Hydrogen is usually employed to manipulate the molecular weight of thepolymer product. The use of hydrogen in such manipulation isconventional in the art. Suitable amounts of hydrogen which may be addedare 0.001 to 100 mol H₂/kmol propylene, preferably in the range 0.5 to30 mol H₂/kmol propylene.

Under conventional polymerisation conditions, i.e. at a temperature lessthan 80° C., typically at 60 to 70° C., higher olefins do notcopolymerise easily. The person skilled in the art knows that thecopolymerisation rate is dependent on the nature of the higherolefin—the longer the monomer chain, the lower the copolymerisationrate. Thus, under such low temperature conditions, in order to obtain apropylene copolymer comprising, for example, 1% by weight of higherα-olefin, e.g. hexene, it is necessary to employ a considerable excessof hexene in the reaction medium. For example, when the Ziegler-Nattacatalyst of EP-A-591224 is used in low temperature polymerisation at,for example, 70° C., in order to obtain 1% wt hexene in a propylenecopolymer you would require 12 wt % hexene in the reaction mediumrelative to propylene.

The use of higher polymerisation temperatures or supercriticalpolymerisation conditions may allow more higher olefin to becopolymerised into a polymer than under lower temperature conditions.

Thus, for example, in order to incorporate 1 wt % of hexene into apolymer at 90° C. would require only 10 wt % of hexene in the feed.Further, by using supercritical conditions only 5.9 wt % of hexene wouldbe required in the reaction mixture. Thus, at least 20% more hexene iscopolymerised into the polymer when higher temperature polymerisationconditions are used. The same kind of behaviour is observed for otherhigher olefins and for octene, the difference is even more marked.

On an industrial scale such significant increases in higher olefinincorporation give rise to significant economic savings for a number ofreasons. Firstly, less higher olefin needs to be employed in thereaction medium to obtain the same amount of comonomer in the polymer asunder low temperature subcritical conditions. Secondly, sinceincorporation of higher olefin is higher, less unreacted comonomerremains in the outlet stream to be removed from the reaction mixture.For obvious economic and environmental reasons, unreacted monomer needsto be carefully removed from the process and obtained polymer andreisolated for future use. Any reduction in the amount of comonomerpresent simplifies the monomer separation and recycling process againallowing the polymer manufacturer to make significant economic savings.Additionally, by using higher reactor temperatures, residual comonomermay be more easily removed from the reactor since little heat isrequired to cause evaporation of the comonomer.

Moreover, since the process of the invention gives rise to polymershaving less residual higher olefin monomer, the taste and odour of theproduced polymers is improved.

The catalyst to be employed in the present invention may be any kind ofcatalyst suitable for use in olefin polymerisations but usually is aconventional Ziegler-Natta catalyst system which is well-known in theart. A typical Ziegler-Natta catalyst system used in the invention is ahighly stereospecific catalyst system comprising a catalyst component(procatalyst), a cocatalyst component and an external donor, thecatalyst component containing as essential elements magnesium, titaniumand halogen. The procatalyst may therefore comprise a titaniumhalogenide compound on a magnesium chloride compound. A typicalcocatalyst is a C₁₋₁₀ aluminum alkyl. The catalyst system shouldadditionally include electron donor compounds such as ethers, esters,silanes and the like. The use of such donors is of course routine.Conventional carrier materials such as MgCl₂ and silica may also beemployed.

Catalysts of use in the invention can be prepared as described in, forexample, EP-A-491566, EP-A-627449, EP-A-889915, EP-A-926165,EP-A-591224, EP-A-586390, WO00/68277 and U.S. Pat. No. 5,234,879 all ofwhich are incorporated herein by reference.

It is important that the catalyst employed works efficiently at thehigher reactor temperatures/supercritical temperature of the presentinvention. Some conventional Ziegler-Natta species for isotacticpolymerisation generally have an operating temperature of around 80° C.above which they lose activity and/or stereoselectivity. Whilst suchcatalysts can of course be employed if adjuvants are used to reduce thesupercritical temperature within the reactor it is preferred if theemployed catalyst operates fully at the preferred reactor temperature.

In this regard an especially preferred catalyst is described inEP-A-591224 or EP-A-491566. In both these applications there isdescribed a method for preparing a procatalyst composition frommagnesium dichloride, a titanium compound, a lower alcohol and an esterof phthalic acid. In order to produce the catalyst species, atransesterification reaction is carried out between the alcohol andphthalic acid ester to give a particularly advantageous Ziegler-Nattacatalyst.

Generally, the quantity of catalyst used will depend upon the nature ofthe catalyst, the reactor types and conditions and the propertiesdesired for the polymer product. Conventional catalyst quantities, suchas described in the publications referred to herein, may be used.

Whilst the process of the invention is primarily for use in thepreparation of propylene polymers with a single other higher olefin, itis also within the scope of the present application to prepareterpolymers, i.e. polymers comprising propylene and at least two otherhigher olefins e.g. hexene and butene or terpolymers comprisingpropylene, ethylene and at least one higher olefin. The use of highreactor temperatures or supercritical conditions should increase theincorporation of all higher olefin species present again allowingeconomic savings to be made.

The polymerisation process of the invention should take place insuspension liquid slurry or bulk where the polymer forms as aparticulate. Preferably, the process of the invention occurs inslurry-bulk where the reaction medium comprises mainly propylene. Aconventional loop reactor or a continuous stirred tank reactor may formthe slurry reactor. Both reactors are well-known in the art. In apreferred embodiment, the reactor should be a loop reactor, especially aloop reactor working in bulk.

The polymerisation may be carried out by feeding the catalyst system, amixture of propylene, comonomer and optionally hydrogen and adjuvantsinto the reactor and circulating the mixture with a pump. Heat ofpolymerisation can be removed using a conventional cooling jacket.

The continuous stirred tank reactor or a loop reactor may form the onlyreactor used in the polymerisation process, however, it is preferred iffurther gas phase/loop or stirred tank reactors are also employed. Saidfurther loop or stirred tank reactors can be operated at lower or highertemperature conditions.

In a particularly preferred embodiment two reactors are employed, thefirst reactor being a loop reactor operating under high temperature(greater than 80° C.)/supercritical conditions) and the second reactorbeing a conventional gas phase reactor. The reaction mixture can betransferred from the loop reactor to the gas phase reactor byconventional techniques using appropriate flashing methods etc ordirectly and gas phase polymerisation can be carried out at atemperature of for example, 60 to 100° C. at a pressure of, for example,5 to 50 bars. The properties of the polymer produced can be manipulatedusing H₂ and/or comonomer as is known in the art.

It is also within the scope of the invention to employ a further gasphase reactor subsequent to the first gas phase reactor.

Another preferred reactor set up involves the use of two loop reactors.At least one of the reactors should be operated according to theinvention, i.e. by employing high temperature polymerisation. It is alsopossible to run both reactors under the conditions required by theprocess of the invention.

In addition to the actual polymerisation reactors used for producing thepropylene copolymer, the polymerisation reaction system can also includea number of additional reactors such as pre and/or post reactors. Thepre-reactors include any reactor for prepolymerising thepreactivated/modified catalyst with propylene and/or other olefin(s).The post-reactors include reactors used for modifying and improving theproperties of the polymer product. All reactors in the reactor systemare preferably arranged in series.

Using the process of the invention it is possible to prepare propylenecopolymers having wide ranging properties. The molecular weight, meltflow index, crystallinity, melting point and melt range of the polymerscan be readily adjusted. The polymer may also have improved mechanicalproperties such as flexural modulus and elasticity and are normallyproduced in particle form.

The copolymers produced by the process of the invention haveapplications in a wide variety of areas, e.g. pipe, film, sheet, fibre,mouldings, wire and cable.

The invention will now be described with reference to the followingnon-limiting examples and Figure, which shows hexene incorporation incopolymer versus polymerisation temperature results for Examples 2, 5and 6 and comparative example 4.

Experimental Methods

The following procedures were used to determine polymer parameters.

MFR

MFR₂:ISO 1133 Standard, at 230° C., using 2.16 kg load

Xylene Soluble Fraction

XS: Determination of xylene soluble fraction (XS): 2.0 g of polymer isdissolved in 250 ml p-xylene at 135° C. under agitation. After 30±2minutes the solution is allowed to cool for 15 minutes at ambienttemperature and then allowed to settle for 30 minutes at 25±0.5° C. Thesolution is filtered with filter paper into two 100 ml flasks.

The solution from the first 100 ml vessel is evaporated in nitrogen flowand the residue is dried under vacuum at 90° C. until constant weight isreached.

The xylene soluble fraction is calculated using the following equation:XS %=(100×m ₁ ×v _(o))/(m _(o) ×V ₁),wherein

-   -   m_(o)=initial polymer amount (g)    -   m₁=weight of residue (g)    -   v₀=initial volume (ml)    -   v₁=volume of analysed sample (ml)        Thermal Properties

Melting temperature, Tm, crystallisation temperature, T_(cr), and thedegree of crystallinity were measured with Mettler TA820 differentialscanning calorimetry (DSC) on 3±0.5 mg samples. Both crystallisation andmelting curves were obtained during 10° C./min cooling and heating scansbetween 30° C. and 225° C. Melting and crystallisation temperatures weretaken as the peaks of endotherms and exotherms. The degree ofcrystallinity was calculated by comparison with heat of fusion of aperfectly crystalline polypropylene, i.e. 209 J/g.

EXAMPLE 1

All raw materials were essentially free from water and air and allmaterial additions to the reactor and the different steps were doneunder inert conditions.

The polymerisation was carried out in a 5 litre reactor, which washeated, vacuumised and purged with nitrogen. 78 μl TEA (triethylaluminium), 13 μl donor D (dicyclopentyldimethoxy silane) and 30 mlpentane were mixed and allowed to react for 5 minutes. Half of themixture was added to the reactor and the other half was mixed with 5.2mg highly active and stereospecific Ziegler Natta catalyst (ZNcatalyst). The ZN catalyst was prepared according to the catalystsynthesis of EP-A-591224, and had Ti content 2.1 wt %. After about 10minutes the ZN catalyst/TEA/donor D/pentane mixture was added to thereactor. The Al/Ti molar ratio was 250 and the Al/Donor molar ratio was10. 140 mmol hydrogen, 50 g 1-hexene and 950 g propylene were added tothe reactor and the temperature was raised to 102° C. over 26 minutes.The pressure in the reactor was 50 bars. The reaction was stopped after1 hour at 102° C. by flashing out unreacted propylene and 1-hexene.2-propanol (5 ml) was added after flashing in order to kill the catalystand prevent further polymerisation of any residual 1-hexene. Finally,the polymer powder was dried in the reactor at 50° C. with nitrogenpurge for 1.5 hours.

The polymer was analysed and the results are shown in table 1 below. Thehexene content in the polymer was 0.85 wt %.

COMPARATIVE EXAMPLE 1

This example was carried out as described in example 1, with theexception that the polymerisation temperature was 80° C., and thepolymerisation pressure was 37 bars. The concentrations of propylene,1-hexene and hydrogen in the liquid phase are approximately the same asin Example 1. The details and results are shown in table 1. The hexenecontent in the polymer was only 0.65 wt %.

EXAMPLE 2

This example was carried out as described in example 1 with theexception that the amount of 1-hexene was doubled and the polymerisationtemperature was 110° C. The details and results are shown in table 1.The hexene content in the polymer was 1.67 wt %.

TABLE 1 Polymerisation conditions and polymer properties Ex 1 Comp Ex 1Ex 2 Conditions Temperature ° C. 102 80 110 Propylene g 950 1330 9001-hexene g 50 73 100 Hexene in wt % 5 ~5.7 10 liquid Hydrogen mmol 140250 180 Hydrogen mol-% 0.6 ~0.6 ~0.8 in liquid Pressure bar 50 37 50Yield g 241 318 242 Activity kgPP/ 46 43 33 gcat Polymer Properties MFRg/10 min 8.5 6.3 17.3 XS wt % 1.3 1.5 1.5 Hexene Wt % 0.85 0.65 1.67 Tm° C. 156.1 157.8 151.6 Tcr ° C. 118.3 118.1 112.9 Crystallinity % 47 4941

It is clear from the examples that the amount of hexene incorporatedinto a polymer is greater when the polymerisation takes place attemperatures greater than 80° C. (Ex 1 and 2) than at lower temperature(Comp. Ex 1) even though under the higher temperature conditions lesshexene was fed into the process.

EXAMPLE 3

This example was carried in accordance with Example 1 with the exceptionthat 1-octene was used as comonomer and the polymerization temperaturewas 100° C. The 1-octene content in the liquid was 2.5 wt. %. Thedetails and results are shown in table 2. The 1-octene content in thepolymer was 0.55 wt. %.

COMPARATIVE EXAMPLE 2

This example was carried out as described in Example 3, with theexception that the temperature was 80° C. The details and results areshown in table 2. The 1-octene content in the polymer was 0.27 wt. %.

EXAMPLE 4

This example was carried described in Example 3, with the exception thatthe amount of 1-octene was doubled and the polymerisation temperaturewas 102–105° C. The details and results are shown in table 2. The1-octene content in the polymer was 0.97 wt. %.

COMPARATIVE EXAMPLE 3

This example was carried out as described in Example 4, with theexception that the polymerisation temperature was 80° C. The details andresults are shown in table 2. The 1-octene content in the polymer was0.31 wt. %.

TABLE 2 Conditions and polymer properties with 1-octene Comp CompExample 3 Ex 2 Example 4 Ex 3 Conditions Temperature ° C. 100 80 102–10580 Pressure bar 51–50 ~39 ~51.5 ~38 Propylene g 1100 1360 1100 1330Octene g 28 32 53 63 Octene in wt % 2.5 ~2.5 4.6 ~5 liquid Hydrogen mmol159 250 146 250 Hydrogen mol-% 0.6 ~0.6 0.55 ~0.6 in liquid Yield g 302292 309 319 Activity kgPP/gcath 51.2 42.3 44.1 44.9 (bulk) PolymerProperties MFR g/10 min 9.2 7.4 12.5 7.3 XS wt % 1.1 1.3 1.2 1.4 Octenewt % 0.55 0.27 0.97 0.31 Tm ° C. 157.9 162.4 155.8 159.2 Tcr ° C. 115.9116.5 113.8 116.5 Crystallinity % 50 51 46 50

EXAMPLE 5

This example was carried as described in Example 1, with the exceptionthat the amount of 1-hexene was doubled, corresponding to about 10 wt. %in the liquid phase. The polymerisation temperature was 90° C. Thedetails and results are shown in table 3. The 1-hexene content in thepolymer was 1.0 wt. %.

EXAMPLE 6

This example was carried out as described in Example 5, with theexception that the polymerisation temperature was 100° C. The detailsand results are shown in table 3. The 1-hexene content in the polymerwas 1.32 wt. %.

COMPARATIVE EXAMPLE 4

This example was carried out as described in Example 5, with theexception that the polymerisation temperature was 70° C. The details andresults are shown in table 3. The 1-hexene content in the polymer was0.83 wt. %.

TABLE 3 Conditions and properties for 1-hexene with 10 wt. % 1-hexene inthe liquid Comp Ex 4 Example 5 Example 6 Conditions Temperature ° C. 7090 100 Pressure bar 31 41 46 Propylene g 1270 1200 1100 1-hexene g 130125 118 1-hexene in liquid wt % ~10 ~10 ~10 Hydrogen mmol 300 210 160Hydrogen in mol-% ~0.6 ~0.6 0.6 liquid Yield g 204 319 306 Activity(bulk) kgPP/gcath 24 53.2 54.6 Polymer Properties MFR g/10 min 14.9 9.813.3 XS wt % 2 1.4 1.5 1-hexene wt % 0.83 1 1.32 Tm ° C. 153.8 152.8152.5 Tcr ° C. 114.8 112.7 113.1 Crystallinity % 47 45 44

1. A single stage or a multistage process for the subcriticalcopolymerisation of propylene with a comonomer selected from 1-hexene,1-octene or a C₄₋₂₀ diene olefin wherein the polymerisation is carriedout in the presence of a Ziegler-Natta catalyst system, at least onestage is carried out at a temperature of greater than 80° C. and whereinall stages are carried out in a slurry reactor.
 2. A process as claimedin claim 1 wherein said comonomer is 1-octene.
 3. A process as claimedin claim 1 wherein said process takes place in bulk.
 4. A process asclaimed in claim 1 wherein said slurry reactor is a loop reactor or astirred tank reactor.
 5. A process as claimed in claim 1 wherein thecopolymerisation takes place at a temperature of at least 85° C.
 6. Aprocess as claimed in claim 5 wherein the polymerisation operates at atemperature of at least 90° C.
 7. A process as claimed in claim 6wherein the polymerisation operates at a temperature of at least 100° C.8. A process as claimed in claim 1 wherein the amount of 1-hexene,1-octene or C₄₋₂₀ diene in the reaction medium is up to 15 wt %.
 9. Aprocess as claimed in claim 8 wherein the amount of 1-hexene, 1-octeneor C₄₋₂₀ diene in the reaction medium is up to 10 wt %.